Method and apparatus for attitude detection and stabilization of an airborne vehicle using the electrostatic field in the earth{3 s atmosphere

ABSTRACT

A method and apparatus for utilizing the earth&#39;&#39;s static electric field as a reference for generating an information-bearing electrical signal. In a particular embodiment, the invention provides a differential static voltmeter having spaced voltage sensing probes and a differential static amplifier which determines the difference in voltage sensed by the probes. Since the probes measure the static voltage which occurs as a vertically oriented gradient in the earth&#39;&#39;s atmosphere, the voltage difference signal produced by the differential static amplifier is a measure of the disposition of the probes relative to a line usually parallel to the earth&#39;&#39;s surface, the voltmeter thereby being particularly useful as a pitch and roll stabilization reference and control system for airborne vehicles. In other embodiments, the invention also provides apparatus for defining a horizontal reference plane in the atmosphere and apparatus for sensing the roll or pitch rate of an airborne vehicle.

United States Patent Hill [451 Feb. 25, 1975 [75] Inventor: Maynard L.Hill, Silver Spring, Md.

[73] Assignee: The Johns Hopkins University,

Baltimore, Md.

[22] Filed: Oct. 27, 1972 [21] Appl. N0.: 301,363

[52] US. Cl. 244/77 R, 317/262 E, 324/32, 324/72, 318/584, 318/585,340/27 AT [51] Int. Cl. B64c 13/18 [58] Field of Search... 244/15 A, 76R, 77 R, 77 D, 244/77 E, 77 SS; 250/336, 374;

158; 340/27 AT; 317/262 E, 201 R, 177 R 8/1972 Casan et a1. 318/588 X3/1973 Gardner 244/77 D Primary ExaminerTrygve M. Blix AssistantExaminer-Stephen G. Kunin Attorney, Agent, or Firm-Robert Archibald;Kenneth E. Darnell [57] ABSTRACT A method and apparatus for utilizingthe earths static electric field as a reference for generating an information-bearing electrical signal. In a particular embodiment, theinvention provides a differential static voltmeter having spaced voltagesensing probes and a differential static amplifier which determines thedifference in voltage sensed by the probes. Since the probes measure thestatic voltage which occurs as a vertically oriented gradient in theearths atmosphere, the voltage difference signal produced by thedifferential static amplifier is a measure of the disposition of theprobes relative to a line usually parallel to the earth's surface, thevoltmeter thereby being particularly useful as a pitch and rollstabilization reference and control system for airborne vehicles. Inother embodiments, the invention also provides apparatus for defining ahorizontal reference plane in the atmosphere and apparatus for sensingthe roll or pitch rate of an airborne vehicle.

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DIFFERENTIAL VOLTAGE AMPLIFIER SERVOMECHANICAL APPARATUS METHOD ANDAPPARATUS FOR ATTITUDE DETECTION AND STABILIZATION OF AN AIRBORNEVEHICLE USING THE ELECTROSTATIC FIELD IN THE EARTHS ATMOSPHERE Theinvention herein described was made in the course of or under a contractwith the Department of the Navy.

BACKGROUND AND SUMMARY OF THE INVENTION The existence of a staticelectric field in the earths atmosphere has long been recognized. Morerecently, this static electric field was determined to exhibit avertical potential gradient, the change in potential per meterdecreasing with increasing altitude. At any altitude, however,equipotential lines and planes are essentially horizontal, especially ina localized area. The ability to define a horizontal equipotential lineor plane in the atmosphere would provide a reference for aircraftstabilization, gyroscopic drift correction, or-,clean air turbulencedetection inter alia. Additionally, detection of tilted equipotentialplanes or lines in the vicinity of various physical objects provides areference for obstaele avoidance apparatus. I

In particular, the present invention provides related methods andpreferred means for utilizing the earths static field existing therein,to obtain a reference signal which can be employed to control both pitchand roll axes stabilization of airborne vehicles. In a general sense,the invention provides for continuously defining an equipotential lineor plane within the earths vertical static potential gradient.Definition of this equipotential line or plane is indicated by a simpleDC voltage signal produced by the preferred apparatus of the invention,this signal being usable to accomplish a variety functions. Theinvention further relates to a general method for continuously definingan equipotential line or plane in the earth's vertical static potentialgradient. The method comprises immersion of at least two voltage sensingprobes in the earth's static potential gradient and measuring thedifference in the static voltage sensed by the two probes. If the twoprobes sense the same voltage, i.e., the difference between the sensedvoltage is zero, then the probes usually lie on a horizontal line ofequal potential due to the vertical nature of the earths potentialgradient. Similarly, a horizontal equipotential plane may be defined bytwo pairs of voltage sensing probes or by three probes, one of which isreferenced'to the electrical center of the other two probes.Nonhorizontal equipotential planes and lines may be sensed in similarfashion in those areas where the vertical gradient is distorted due tophysical objects or ionization sources.

The preferred apparatus of the present invention comprises voltagesensing probes (which probes may be comprised of radioactive material inorder to increase the electrical contact thereof with the atmosphere)and a differential static amplifier. The amplifier receives the sensedpotentials from the probes and measures the difference in the sensedpotentials to produce a voltage difference signal which maybe utilizedto control a servomechanical or other apparatus. The voltage sensingprobe/differential static amplifier combination is referred tohereinafter as a differential static voltmeter. The actual or effectivecombination of two differential static voltmeters can essentially definean equipotential plane in the earths vertical static potential gradient.Since this equipotential plane is defined within a relatively smallspace, the plane is essentially horizontal and may be used as areference plane for aircraft stabilization inter alia.

If the sensing probes described above are attached to the wingtips of anaircraft or essentially along the transverse axis thereof, then thevoltage difference between the probes measured by the differentialstatic amplifier is a function of the roll angle of the aircraft.Similarly, the voltage difference sensed by a pair of sensing probesdisposed essentially along the longitudinal axis of an aircraft is afunction of the pitch angle of the aircraft. The differential voltageoutput signals thus produced for the pitch and roll orientations of theaircraft can be used to drive a servomechanical system to maintain theaircraft in level flight relative to the surface of the earth. Thesensing probes need not be exactly aligned with or parallel to thetransverse or longitudinal axes of the aircraft in order to produceuseful information. Similarly, the probes need not be positioned atexactly the same elevation relative either to the earths surface or tothe horizontal axis of the aircraft.

The invention also relates to a method and apparatus for measuring theroll rate or pitch rate of an airborne vehicle. The apparatus used tosense rate comprises a pair of conductive wires disposed essentiallyalong or parallel to the transverse axis or longitudinal axis of theaircraft. Rotation of these wires in the earths static electric fieldgenerates the current in the wires. The generated current is measured bya current meter, the intensity of the current being proportional to theroll rate or pitch rate of the vehicle.

Thus it is an object of the invention to provide a method and apparatusfor continuously defining an equipotential line or plane in the earthsstatic electric field.

It is another object of the invention to provide differential voltagesensing apparatus aboard an airborne vehicle, the differential voltagesensed by said apparatus being a function of the roll or pitch attitudeangle of the vehicle.

It is a further object of the invention to provide a dif ferentialstatic current meter for determining the roll rate or pitch rate of anairborne vehicle by measuring the current induced in a pair ofconductive wires rotated in the earths static potential gradient.

Further objects and advantages of the invention will become more readilyapparent in light of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of adifferential static voltmeter mountable on an aircraft;

FIG. 2 is a detailed diagram of the circuitry of the voltmeter of FIG.1;

FIG. 3 is a schematic view of an aircraft illustrating roll orientationsof the aircraft;

FIG. 4 is a schematic view of an aircraft illustrating pitchorientations of the aircraft;

FIG. 5 is an idealized schematic view of an equipotential plane detectorcomprised of two differential static voltmeters;

FIG. 6 is a schematic view of an aircraft fitted with an equipotentialplane detector comprised of three voltage sensing probes, the detectorbeing shown in part as a detailed circuit diagram;

FIG. 7 is an idealized schematic view of a equipotential plane detectorhaving additional'voltage sensing probes for automatic gain control;and,

FIG. 8 is a detailed diagram of the circuitry employed to determine theroll rate of an airborne vehicle.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The several embodiments of theinvention use the earths static potential gradient as a reference forgenerating an information-bearing electrical signal. The generatedsignal may then be used to perform a variety of functions, a singularlyvaluable such function being the stabilization of an airborne vehicle.The invention essentially provides a method for measuring the differencein the earths static potential field between two closely spaced pointstherein. In a simple form of the invention, a differential staticvoltmeter is utilized to generate a signal proportional to themisalignment of the axis joining two spaced sensors of the voltmeterwith a line taken parrallel to the surface of the earth. An effectivecombination of two differential static voltmeters can be used to definean equipotential plane within the earths static potential gradient. Theequipotential plane thus defined is usually essentially parallel to theearths surface, thereby creating an artificial horizontal plane whichcan then be utilized for stabilization of an airborne vehicle in bothpitch and roll modes. The invention thus contemplates utilization of theearths static potential gradient to provide roll attitude and roll rateand/or pitch attitude and pitch rate information in the form of aneasily applied DC signal.

Referring to FIGS. 1 and 2 a differential static voltmeter is seen at 10to comprise sensing probes 12 and a differential voltage amplifier 14,the probes 12 being mounted for illustration on the wingtips of anaircraft 16. The output signal of the voltmeter l0, i.e., essentiallythe output signal of the amplifier 14, is directed to well-knownservomechanical apparatus 18 to accomplish a desired function, such asto ailerons 20' to provide roll stabilization. The sensing probes 12 maycomprise metal pads 22 coated with a layer 24 of a radioactive metal ormetal salt toincrease the conductivity of the air in the vicinity of theprobes by alpha particle emission (and resulting ionization of the air),thereby to improve the electrical contact of the probes with the air.The probes 12 may simply comprise con-. ductive needle-like points orthin pointed metal wires, chemical sprays, hot gases, etc. The probes 12respond rapidly to changes in the static voltage encountered in theatmosphere. The voltage sensed by the probes 12 produces a low impedanceinput to the differential voltage amplifier 14.

As can be particularly seen in FIG. 2, the probes 12 are electricallyjoined to the amplifier 14 by either coaxial cables or other electricalconnections 26 to maintain a high insulation resistance in the probewiring to avoid leakage paths. The differential voltage amplifier 14comprises two operational amplifiers 28a and 28b, the amplifiers 28a and28b receiving the inputs from one of each of the probes 12. Inserted inthe circuit between each of the probe/amplifier combinations areresistors 30a and 30b joined in series to potentiometers 32. Theresistors 30a and 30b cause a high impedance input into the operationalamplifiers 28a and 28b. The output from amplifier 28a is fed throughresistor 34 into the negative terminal of the amplifier 28b, thepositive terminal of the amplifier 28b being received directly from theresistor 30b. The output signal resulting from the amplifier 281), thusthe signal generated within the differential voltage amplifier 14, isproportional to the voltage differential sensed by the two probes 12. Ifthe probes 12 are level with respect to an ambient equipotential line orplane, a zero differential voltage results. Elevation of one of theprobes 12 above the other probe produces a differential voltage signalproportional to the difference in elevation between the probes, thevoltage difference resulting from the vertical static voltage gradientexisting in the earths atmosphere. The differential static voltmeter 10can then be seen to be capable of performing a variety of functions, oneof which being the stabilization in pitch or roll of an airbornevehicle. By disconnecting the probes 12 from the amplifier 14, theresistor 30a may be adjusted to zero volts at 36 and the resistor 30badjusted to a desired output which represents a neutral horizontalgradient. Placement of one of the probes 12 on the opposite wingtips ofthe aircraft 16 results in a static voltm eter output which isproportional to the bank angle of the aircraft 16. Similarly, a staticvoltmeter 10 having its probes 12 mounted effectively along thelongitudinal axis of the aircraft 16 produces an output proportional tothe dive or climb angle of said aircraft. The output signals from thestatic voltmeters 10 so mounted on the aircraft 16 can then be filteredand fed to the servomechanical apparatus 18. Pitch or roll stabilizationmay thus be accomplished. A combination of two static voltmeters 10 maybe used to stabilize an airborne vehicle in both pitch and roll.

FIG. 3 shows three aircraft a, 50b, and 50c fitted with staticvoltmeters and operating in the static voltage field E, the probes 12being visible on the wingtips of the aircraft: The field E is seen toexhibit a vertical gradient, the static potential per meter in theatmosphere decreasing with altitude. Thus, the lines 52 forming thestatic voltage field E are shown as parallel lines which are closertogether near the earths surface 54. The lines 52 are distorted andtilted about mountains 57 or other such obstacles. The aircraft 50a isseen to be operating in level flight the wingtips of the aircraft beingaligned along a line 56 which is parallel to the earths surface 54. Theprobes 12, which need not be fitted on the wingtips of the aircraft 50abut could be spaced closely together within a small-unit mountedvirtually anywhere on the aircraft, are aligned essentially along thehorizontal line 56. However, greater mutual spacing of the probes 12generally yields greater sensitivity to attitude misorientation of theaircraft. The probes 12 sense the same static voltage while in the rollattitude illustrated by the aircraft 50a, the voltmeter joined to theprobes l2 producing a zero differential voltage output signal whichindicates that the aircraft 50a has a zero roll angle. The aircraft 50bis shown to have a left bank roll orientation, i.e., the left wingtip ofthe aircraft is lower relative to the earths surface than the rightwingtip. In such an orientation, a differential voltage output signal isproduced which is proportional to the difference in elevation of theprobes 12 on the wingtips. This difference in probe elevation isgeometrically proportional to the roll angle 0 defined by theintersection of the horizontal line 56 and the straight line joining theprobes 12. Similarly, the aircraft 50c has a right bank rollorientation, the difference in elevation between the probes 12 on thewingtips of the aircraft producing a voltmeter output which isproportional to the angular roll orientation of the aircraft. Bydefining the left bank orientation of the aircraft 50b.as a positiveroll angle and the right bank orientation of the aircraft 500 as anegative'roll angle, the output of the voltmeter 10 can provide a pilotwith an indication not only of the degree of the roll angle but also ofthe direction. As has previously been described, the output of thedifferential static voltmeter may be fed into well-known closed-loopservomechanical apparatus for automatically maintaining the aircraft 50bor 500 in a horizontal orientation.

FIG. 4 also shows the aircraft 50a, 50b, and 50c in the earths staticvoltage field E, a side elevational view of the air-craft being shownsuch that the pitch angle 4) of the aircraft relative to a horizontalline 60 may be observed. The longitudinal axis of the aircraft 50a isaligned parallel with the horizontal line 60, thereby being alsoparallel to the earths surface 54, the probes 12 being disposedessentially along the longitudinal axis of the aircraft 50a. The probes12 thus sense the same static voltage and the voltmeter to which theprobes 12 are connected produces a zero differential voltage outputsignal which indicates that the aircraft has a zero pitch angle. Theaircraft 50b is seen to be diving, or pitched downwardly, the probes 12on the fore and aft of the aircraft 50b thereby having a differentelevation. The probe 12 on the front of the aircraft 50b senses a morenegative static voltage than does the probe 12 toward the rear of theaircraft. This sensed difference in voltage is reflected in the outputsignal of the differential static voltmeter to which the probes 12 areelectrically connected, which output signal is proportional to the pitchangle formed by the intersection ofthe horizontal line 60 and thelongitudinal axis of the aircraft. Similarly, the aircraft 500 is seento be climbing, or pitched upwardly, the difference in elevation betweenthe probes 12 on the aircraft producing a voltmeter output which isproportional to the angular pitch orientation of the aircraft. Thevoltmeter output may then be used to inform servomechanical apparatus ofthe magnitude and direction of the pitch angle 4: of the aircraft sothat the aircraft can be automatically stabilized.

Use of a pair of differential static voltmeters 10 such as isschematically shown in FIG. 5 allows definition of an equipotentialplane within the static potential gradient in the atmosphere. Each pairof probes 12 in FIG. 5 may be considered to lie along a line or axisjoining said probes. If these axes are turned to angles to each othersuch as the 90 angles shown in FIG. 5, then each voltmeter will definean equipotential line when the output thereofis zero. When the output ofboth voltmeters 10 is zero, then the plane in which the four probes l2lie would have a substantially equal static potential value at everypoint in the plane. Due. to the relative localized measurement made bythe voltmeters 10, the equipotential plane is usually parallel to thesurface of the earth, thereby defining an artificial horizontal planewhich may be used as a reference plane for a number of functionsincluding aircraft stabilization, gyroscopic drift adjustment,turbulence detection and unmanned aircraft guidance. Each pair of probes12in each voltmeter 10 is electrically connected to a differentialvoltage amplifier 14 such as has previously been described. The outputof each of the amplifiers 14 may be used to accomplish a desiredfunction. For example, the output signals of the voltmeters 10 of FIG. 5could be used to maintain an aircraft in level flight, one of saidvoltmeters 10 providing a signal to control servomechanical apparatusmaintaining a zero pitch angle and the second voltmeter l0 maintaining azero roll angle. Thus, the aircraft would be maintained in level flightwithin the equipotential plane defined by the pair of voltmeters 10.When the static potential gradient is tilted due to obstacles such asmountains, (as can be seen in FIG. 3), detection of the tiltedequipotential plane is useful in preventing an airborne vehicle fromcolliding with the obstacle. For example, an aircraft stabilized inhorizontal flight with respect to an essentially level portion of theearths surface is capable of avoiding a mountain or hill which tilts thepotential gradient. By sensing the tilted equipotential plane caused bythe obstacle and stabilizing the aircraft with respect to the tiltedequipotential plane, the aircraft automatically assumes the properheading to fly over or around the obstacle.

Simultaneous stabilization in pitch and roll may also be accomplishedusing the system shown in FIG. 6. Three probes 12 are located on anaircraft 65, the aircraft incidentally being of a Delta-wingconfiguration. Two of the probes 12 are disposed along a line toward therear of the aircraft 65, the third probe 12 being located on the forwardportion of the aircraft. The forward probe 12 need not lie in the samehorizontal plane as do the other two probes 12 since, as is the casewith the several voltmeter stabilization units described above, thedifferential voltage amplifiers 14 may be adjusted to a zero staticvoltage reading such that all of the probes 12 need not lie in theequipotential plane being defined. The two rearwardly disposed probes 12are used for roll control and the forwardly disposed probe 12 isreferenced to the electrical center of the rear probes 12 to providepitch control. Essentially, the rear probes l2 combine with adifferential voltage amplifier as has previously been described tocontrol the roll stability of the aircraft 65 by feeding an outputsignal to a roll servomechanical apparatus 72. Since the sum of thevoltages sensed by the rear probes 12 with respect to the center pointis essentially zero as a function of pitch, the forward probe 12 isreferenced to this voltage in order to determine the pitch angle of theaircraft. In essence, the forward probe 12 is referenced to theelectrical center of the rear probes 12, the rear probes 12 functioningas the equivalent of an extra probe 12 which changes in total potentialwith changes in elevation independently of whether the air-craft islevel or in a bank orientation. In this fashion, an output signalproportional to the pitch angle of the aircraft 65 is obtained and isfed to a pitch servomechanical apparatus 74 for correction in order tomaintain a desired pitch orientation.

Use of the voltmeter pair as described relative to FIG. 5 or thethree-probe system shown in FIG. 6 as an aircraft stabilization systemrequires provision for signal gain adjustment at altitudes greater thana few thousand feet. For example, the amplitude of the signals obtainedfrom a voltmeter 10 at 10,000 feet above the earths surface is decreasedfive-fold relative to signal amplitude at an altitude of 3,000 feet.This decrease in signal strength occurs due to the change in the staticpotential gradient with respect to altitude, the static potentialgradient at sea level typically being about volts/meter and as low as 10to 20 volts/meter at an altitude of 20,000 feet. Thus, signal adjustmentin the form of gain control is necessary to compensate for the decreasedmagnitude of the static voltage gradient at relatively high altitude.While gain control may be provided in a number of well-known ways, anextra pair of probes 12c, such as shown in FIG. 7, may be used to' sensethe actual magnitude of the static potential gradient at the altitude atwhich an aircraft is operating, this information then being used toadjust the gain of any feedback signals being derived from theequipotential plane sensor comprised of the two voltmeters 10. Theprobes 120 are mounted on a vertical axis with respect to the horizontalaxes joining the probes 12 of the voltmeters 10. The probes l2c arespaced apart vertically at a suitable distance so as to obtain aninsignificant signal to noise ratio, the spacing being on the order of afoot or more. The probes 12c continuously sense the magnitude of thelocal potential gradient which is then measured by a voltmeter 80, theoutput signal of said voltmeter being used to actuate well-known gaincontrol electronics 82. In addition to compensating for signal strengthloss at altitude, the probes 12c lying along a vertical axis would alsorespond to small local variations in the static potential gradient whichoften occur due to atmospheric inversions or excess atmosphericionization. Thus, addition of the probes 12c to the equipotential planesensor comprises of the voltmeters 10 would improve the signal-to-noiseratio of the equipotential plane sensor. In addition, the probes 120combined with the voltmeter 80 can be used to measure local variationsin the static potential gradient which can be indicative of radioactivematerial on or below the surface of the earth, pollution sources, etc.

The device shown in FIG. 7 provides an instantaneous, accurate mechanismfor automatic gain control. In most applications, an average value ofthe static potential gradient is adequate for providing gain control.Thus, it would not usually be necessary to mount the probes 12c on agimballed-feedback-servo-mechanical platform to maintain the probes 12cin a vertical orientation. Further, only one sensing probe 12c locatedabove or below the equipotential plane defined by the voltmeters 10 maybe used for gain control. Essentially, the output of this single sensingprobe 120 would be compared to the zero output of the equipotentialplane defined by the voltmeters 10.

The invention may also be extended to the fabrication of a roll rate orpitch rate sensing such as is shown at 100 in FIG. 8. The device 10senses roll or pitch rate based on the physical principle that adisplacement current is induced in an electrically conductive rod orwire on rotation of same in an electric field. The device 100 has twoconductive rods 102 which may be disposed on the wingtips of anaircraft. Although the rods 102 are exposed to a continuously changingvoltage field, a static electric field cannot exist in the rods. Thus,as displacement (rotation) of the rods occur, electric charges flow fromone rod 102 to the other rod. A current meter 104 inserted in thecircuit between the rods 102 measures the current flowing between therods 102, the direction of the current indicating the direction of theroll or pitch of an aircraft and the magnitude of the current indicatingthe rate of the roll or pitch.

The current meter 104 produces an output Eo which is a voltage signalproportional to the current flowing between the rods 102. Operationalamplifiers 106 convert the current flow in the circuit to a voltagesignal while providing an effective short circuit between the rods 102.Operational amplifier 108 essentially functions as a differentialvoltmeter to measure the difference in potential between the amplifiers106. Capacitors 110 are used to eliminate 60 cycle interference commonlyencountered in the atmosphere rear power sources, etc. The output E0 ofthe current meter 104 may be applied to a conventional servomechanicalapparatus (not shown) in read out devices to control the roll or pitchrate of an aircraft or to inform the pilot. Mounting of the rods 102 onthe wingtips or about the transverse axis of an aircraft yields rollrate information while mounting said rods on or about the longitudinalaxis of the aircraft yields pitch rate information.

Usual practice in autopilot stabilization involves the combination ofattitude and rate information. In the present invention, either thevoltmeter 10 of FIG. 1 inter alia and the rate sensing device of FIG. 8can be modified to produce an output signal which is a combination ofboth attitude and roll rate information. This combination of informationmay be accomplished by adjusting the input impedance of the particularcircuit, the length of the electrical conductors connected to thesensing probes 12, or the intensity of radiation from the sensing probesif the probes are radioactive. In practice, the voltmeter 10 of FIG. 2produces some rate information even as the device 100 of FIG. 8 producessome attitude information.

The invention disclosed herein may be practiced with modification to thestructures and methods outlines above without departing from the scopeof the invention as recited in the appended claims. For example, theprobes 12 may be disposed at an angle to the roll axis or pitch axis ofan airborne vehicle being stabilized according to the invention. Theangle at which a line joining the probes 12 may make with the roll axisor pitch axis can approach 45. Well-known calibration techniques can beemployed to adjust the voltmeter 10 to compensate for this angularmisalignment.

I claim:

1. A method for stabilization relative to the surface of the earth of atleast one axis of an airborne vehicle operating in the static electricfield existing in the earths atmosphere, comprising the steps of:

sensing the electrostatic potential at each of two spaced pointsdisposed substantially along a line extending in the same direction asthe axis about which the vehicle is to be stabilized;

generating a potential difference signal proportional to the magnitudeof the difference in the two sensed potentials, the magnitude of thesignal being proportional to the angular misalignment of the aforesaidaxis and an equipotential line intersecting said axis; and,

angularly displacing the position of the axis of the vehicle to alignwith the aforesaid equipotential line in response to the magnitude ofthe potential difference signal.

2. The method of claim 1 and further comprising the steps of:

measuring the magnitude of the potential gradient existing in the staticelectric field; and,

adjusting the gain of the potential difference signal in proportion tothe magnitude of the measured potential gradient.

3. A method for stabilization along the pitch and roll axes of anairborne vehicle operating in the static electric field existing in theearths atmosphere, comprising the steps of:

sensing the electrostatic potential at each point of two non-linearpairs of spaced points disposed on the airborne vehicle, a first pair ofpoints lying along a line extending in essentially the same direction asthe longitudinal or pitch axis of the vehicle and a second pair ofpoints lying along a line extending in essentially the same direction asthe transverse or roll axis of the vehicle;

generating a potential difference signal proportional to the magnitudeof the difference in the potential sensed at each point in each pair ofpoints, the magnitude of the difference signal generated between thepoints in the first pair of points being proportional to the pitch angleof the vehicle and the magnitude of the difference signal generatedbetween the points in the second pair of points being proportional tothe roll angle of the vehicle; and,

angularly displacing both the pitch and roll axis of the vehicle toreduce the respective potential difference signals to zero, thedisplacements being responsive to the magnitude of the respectivepotential difference signals.

4. The method of claim 3 and further comprising the steps of:

measuring the magnitude of the potential gradient existing in the staticelectric field; and,

adjusting the gain of the respective potential difference signals inproportion to the magnitude of the measured potential gradient.

5. A method for stabilization of at least one axis of an airbornevehicle operating in the static electric field existing in the earthsatmosphere, comprising the steps of:

defining an equipotential line existing in the static electric fieldpresent in the earths atmosphere; and,

aligning the axis of the airborne vehicle with the equipotential line.

6. A method for stabilization along the pitch and roll axes of anairborne vehicle operating in the static electric field existing in theearths atmosphere, comprising the steps of:

defining an equipotential plane existing in the static electric fieldpresent in the earths atmosphere; and,

aligning the pitch and the roll axes of the vehicle with theequipotential plane.

7. A method for stabilization along the pitch and roll axes of anairborne vehicle operating in the static electric field existing in theearths atmosphere, comprising the steps of:

sensing the electrostatic potential at each of at least three non-linearpoints disposed on the airborne vehicle;

generating a potential difference signal proportional to the magnitudeof the difference in potential between at least two of the points, thetwo points lying along a line extending in essentially the samedirection as one of the axes of the vehicle, the magnitude of thedifference signal being proportional to the angular misalignment of thesaid axis and an equipotential line intersecting said axis;

angularly displacing the position of the aforesaid axis to align withthe aforesaid equipotential line in response to the magnitude of thegenerated potential difference signal;

generating a second potential difference signal proportional to themagnitude of the difference inpotential between a third point and theline joining the aforementioned two points; and,

5 displacing the third point relative to the line joining theaforementioned two points to reduce the second potential differencesignal to zero, the displacement being responsive to the magnitude ofthe second potential difference signal.

8. A method for stabilization along at least one axis of an airbornevehicle operating in the static electric field existing in the earthsatmosphere, comprising the steps of:

disposing at least two electrically conductive members on the vehicleabout an axis thereof, the members being electrically connected;

generating a signal proportional to the current flowing between theconductive members on displacement of the vehicle about the axis, themagnitude of the current flow being proportional to the rate of angulardisplacement of the vehicle about the axis; and,

damping the rate of angular displacement of the vehicle about theaforesaid axis to inhibit displacement of the vehicle about said axis.

9. Apparatus for stabilization about at least one axis of an airbornevehicle operating in the electric field existing in the earthsatmosphere, comprising:

at least two static voltage sensing probes disposed on the airbornevehicle;

means electrically connected to the probes for receiving the potentialssensed by said probes and for producing an output signal proportional tothe difference in sensed potential between the probes; and,

means for adjusting the position of at least one of the probes to causethe output signal to be zero, the line joining the probes thereby beingaligned along an equipotential line.

ll). The apparatus of claim 9 wherein the probes are disposed along aline parallel to the roll axis of the airborne vehicle.

11. The apparatus of claim 9 wherein the probes are located along a linedisposed at an angle to the roll axis of the airborne vehicle.

12. The apparatus of claim 11 wherein the angle is less than 45.

13. The apparatus of claim 9 wherein the probes are disposed along aline parallel to the pitch axis of the airborne vehicle.

14. The apparatus of claim 9 wherein the probes are located along a linedisposed at an angle to the pitch axis of the airborne vehicle.

15. The apparatus of claim 14 wherein the angle is less than 45.

16. The apparatus of claim 9 wherein said firstmentioned means comprisesa differential amplifier.

17. Apparatus for stabilization relative to the surface of the earthabout the pitch and roll axes of an airborne vehicle operating in thestatic electric field existing in the earths atmosphere, comprising:

two pairs of static voltage sensing probes disposed on 6 the airbornevehicle, one said pair being disposed along a line substantiallyparallel to the roll axis of the vehicle and the other pair beingdisposed along a line substantially parallel to the pitch axis of thevehicle, the probes in each pair of probes being disposed at spaced,co-planar points on the vehicle;

two differential amplifiers, one each being electrically connected toone pair of probes for receiving the potentials sensed by the probes ofeach pair of probes, each said amplifier producing an output signalwhich is proportional to the difference in sensed potential between thesensing probes of each pair of probes; and,

means for adjusting the position of at least one of the probes in eachpair of probes to cause the output signal of the amplifiers respectivelyconnected thereto to be zero, the vehicle thus being stabilized relativeto the equipotential plane in which the probes lie when the outputsignals of both amplifiers are zero.

18. The apparatus of claim 17 and further comprising:

a pair of vertically disposed static voltage sensing probes, one probeof the pair being disposed above the plane containing the fouraforementioned probes and the other probe being disposed below theaforesaid plane;

means for receiving the potentials sensed by the vertically disposedprobes and for producing an output signal proportional to the staticvoltage gradient between the probes; and,

gain control adjustment means responsive to the lastmentioned outputsignal to provide automatic control of the gain required to render thefirstmentioned output signals of sufficient magnitude to actuate themeans for adjusting the position of the probes in each pair of probes.

19. Apparatus for detecting the attitude relative to the surface of theearth of an airborne vehicle operating in the electrostatic fieldexisting in the earths atmosphere, comprising:

probe means mounted on said vehicle and responsive to the magnitude ofsaid electrostatic field at a first preselected point relative to asecond preselected point,

said first and second preselected points being spaced apart and defininga reference line on said vehicle; and,

means connected to said probe means for producing an output signalproportional to the difference in relative magnitude of saidelectrostatic field at said first and second preselected points,

said output signal indicating misalignment between said reference lineand an equipotential line existing in said earths electrostatic field.

20. The apparatus of claim 19 further including stabilizing means onsaid vehicle responsive to said output signal for adjusting the attitudeof said vehicle to cause said output signal to be zero and thereby alignthe reference line on said vehicle with said equipotential line.

21. The apparatus specified in claim 20,

wherein said probe means comprises two pairs of static voltage sensingprobes disposed on the airborne vehicle, one said pair being disposedalong a line substantially parallel to the roll axis of the vehicle andthe other pair being disposed along a line substantially parallel to thepitch axis of the vehicle, the probes in each pair of probes beingdisposed at spaced, substantially horizontal co-planar points on thevehicle;

wherein said output signal producing means comprises two differentialamplifiers, one each being electrically connected to one pair of probesfor receiving the potentials sensed by the probes of each pair ofprobes, each said amplifier producing an output signal which isproportional to the difference in sensed potential between the sensingprobes of each pair of probes; and,

wherein said stabilizing means comprises means for adjusting theposition of at least one of the probes in each pair of probes to causethe output signal of the amplifiers respectively connected thereto to bezero, the vehicle thus being stabilized relative to the equipotentialplane in which the probes lie when the output signals of both amplifiersare zero.

22. The apparatus of claim 21 and further comprismg:

a pair of vertically disposed voltage sensing probes,

one probe of the pair being disposed above the substantially horizontalplane containing the four aforementioned probes and the other probebeing disposed below the aforesaid substantially horizontal plane;

means for receiving the potentials sensed by the vertically disposedprobes and for producing an output signal proportional to the staticvoltage gradient between the probes; and,

gain control adjustment means responsive to the output signalproportional to said static voltage gradient to provide automaticcontrol of the gain required to render the output signals produced bysaid differential amplifiers of sufficient magnitude to actuate themeans for adjusting the position of the probes in each pair of probes.

23. The apparatus specified in claim 19 wherein said probe meanscomprises at least first and second electrostatic voltage sensor probesmounted on said vehicle at said first and second preselected pointsrespectively.

24. The apparatus of claim 23 wherein the probes are disposed along aline parallel to the roll axis of the airborne vehicle.

25. The apparatus of claim 23 wherein the probes are located along aline disposed at an angle to the roll axis of the airborne vehicle.

26. The apparatus of claim 25 wherein the angle is less than 45.

27. The apparatus of claim 23 wherein the probes are disposed along aline parallel to the pitch axis of the airborne vehicle.

28. The apparatus of claim 23 wherein the probes are located along aline disposed at an angle to the pitch axis of the airborne vehicle.

29. The apparatus of claim 28 wherein the angle is less than 45.

30. The apparatus specified in claim 19 wherein said output signalproducing means comprises a differential amplifier.

31. A method for detecting the attitude relative to the surface of theearth of at least one axis of an airborne vehicle in the static electricfield existing in the earth's atmosphere, comprising the steps of:

defining an equipotential line in the electrostatic field existing inthe earths atmosphere, which line is substantially parallel to thesurface of the earth; and,

generating a signal indicative of misalignment between the axis of saidvehicle and said equipoten- 13 tial line, and thus the misalignmentbetween the axis of the vehicle and the surface of the earth. 32. Themethod specified in claim 31 and further including the step of aligningsaid axis of said airborne vehicle with said equipotential line.

33. The method specified in claim 31 and further including the steps of:

defining a second equipotential line existing in the electrostatic fieldto define an equipotential plane existing in said electrostatic field;and,

aligning the pitch and roll axes of the vehicle with said equipotentialplane.

34. The method specified in claim 31 wherein the step of defining saidequipotential line includes the step of:

sensing said electrostatic field with probe means responsive to therelative magnitude of the electrostatic field at two spaced pointsdefining said vehicle axis.

35. The method specified in claim 34,

wherein the step of sensing the relative magnitude of said electrostaticfield comprises the step of sensing the electrostatic potential at eachof two spaced points disposed along a substantially horizontal lineextending in the same direction as the axis about which the vehicle isto be stabilized; and,

wherein the step of generating said misalignment indicating signalcomprising the step of generating a potential difference signalproportional to the magnitude of the difference in the two sensedpotentials, the magnitude of the signal being proportional to theangular misalignment of the aforesaid axis and an equipotential lineintersecting said axis.

36. The method as specified in claim 35 and further comprising the stepsof:

measuring the magnitude of the vertical potential gradient existing inthe static electric field; and, adjusting the gain of the potentialdifference signal in proportion to the magnitude of the measuredvertical potential gradient. 37. The method as specified in claim 34,and further comprising the steps of:

disposing at least two electrically conductive members on the vehicleabout the axis thereof, the members being electrically connected; and,

generating a signal proportional to the current flowing between theconductive members on displacement of the vehicle about the axis, themagnitude of the current flow being proportional to the rate of angulardisplacement of the vehicle about the axis.

38. A method for detecting the attitude relative to the surface of theearth of at least one axis of an airborne vehicle operating in thestatic electric field existing in the earths atmosphere, comprising thesteps of:

sensing the electrostatic potential at a first point in the atmosphere;

locating a second point in the atmosphere having the same electrostaticpotential as the first point to define an equipotential line in thestatic electric field, the equipotential line being substantiallyparallel to the surface of the earth; and,

generating a signal indicative of misalignment between the axis of thevehicle and the equipotential line, and thus the misalignment betweenthe axis of the vehicle and the surface of the earth.

39. The method of claim 38 wherein the two points are disposedsubstantially along the line extending in the same direction as the axisabout which the vehicle is to be stabilized.

40. The method of claim 38 and further comprising the step of:

aligning the axis of the airborne vehicle in response to the generatedsignal with the equipotential line thus defined.

1. A method for stabilization relative to the surface of the earth of atleast one axis of an airborne vehicle operating in the static electricfield existing in the earth''s atmosphere, comprising the steps of:sensing the electrostatic potential at each of two spaced pointsdisposed substantially along a line extending in the same direction asthe axis about which the vehicle is to be stabilized; generating apotential difference signal proportional to the magnitude of thedifference in the two sensed potentials, the magnitude of the signalbeing proportional to the angular misalignment of the aforesaid axis andan equipotential line intersecting said axis; and, angularly displacingthe position of the axis of the vehicle to align with the aforesaidequipotential line in response to the magnitude of the potentialdifference signal.
 2. The method of claim 1 and further comprising thesteps of: measuring the magnitude of the potential gradient existing inthe static electric field; and, adjusting the gain of the potentialdifference signal in proportion to the magnitude of the measuredpotential gradient.
 3. A method for stabilization along the pitch androll axes of an airborne vehicle operating in the static electric fieldexisting in the earth''s atmosphere, comprising the steps of: sensingthe electrostatic potential at each point of two non-linear pairs ofspaced points disposed on the airborne vehicle, a first pair of pointslying along a line extending in essentially the same direction as thelongitudinal or pitch axis of the vehicle and a second pair of pointslying along a line extending in essentially the same direction as thetransverse or roll axis of the vehicle; generating a potentialdifference signal proportional to the magnitude of the difference in thepotential sensed at each point in each pair of points, the magnitude ofthe difference signal generated between the points in the first pair ofpoints being proportional to the pitch angle of the vehicle and themagnitude of the difference signal generated between the points in thesecond pair of points being proportional to the roll angle of thevehicle; and, angularly displacing both the pitch and roll axis of thevehicle to reduce the respective potential difference signals to zero,the displacements being responsive to the magnitude of the respectivepotential difference signals.
 4. The method of claim 3 and furthercomprising the steps of: measuring the magnitude of the potentialgradient existing in the static electric field; and, adjusting the gainof the respective potential difference signals in proportion to themagnitude of the measured potential gradient.
 5. A method forstabilization of at least one axis of an airborne vehicle operating inthe static electric field eXisting in the earth''s atmosphere,comprising the steps of: defining an equipotential line existing in thestatic electric field present in the earth''s atmosphere; and, aligningthe axis of the airborne vehicle with the equipotential line.
 6. Amethod for stabilization along the pitch and roll axes of an airbornevehicle operating in the static electric field existing in the earth''satmosphere, comprising the steps of: defining an equipotential planeexisting in the static electric field present in the earth''satmosphere; and, aligning the pitch and the roll axes of the vehiclewith the equipotential plane.
 7. A method for stabilization along thepitch and roll axes of an airborne vehicle operating in the staticelectric field existing in the earth''s atmosphere, comprising the stepsof: sensing the electrostatic potential at each of at least threenon-linear points disposed on the airborne vehicle; generating apotential difference signal proportional to the magnitude of thedifference in potential between at least two of the points, the twopoints lying along a line extending in essentially the same direction asone of the axes of the vehicle, the magnitude of the difference signalbeing proportional to the angular misalignment of the said axis and anequipotential line intersecting said axis; angularly displacing theposition of the aforesaid axis to align with the aforesaid equipotentialline in response to the magnitude of the generated potential differencesignal; generating a second potential difference signal proportional tothe magnitude of the difference inpotential between a third point andthe line joining the aforementioned two points; and, displacing thethird point relative to the line joining the aforementioned two pointsto reduce the second potential difference signal to zero, thedisplacement being responsive to the magnitude of the second potentialdifference signal.
 8. A method for stabilization along at least one axisof an airborne vehicle operating in the static electric field existingin the earth''s atmosphere, comprising the steps of: disposing at leasttwo electrically conductive members on the vehicle about an axisthereof, the members being electrically connected; generating a signalproportional to the current flowing between the conductive members ondisplacement of the vehicle about the axis, the magnitude of the currentflow being proportional to the rate of angular displacement of thevehicle about the axis; and, damping the rate of angular displacement ofthe vehicle about the aforesaid axis to inhibit displacement of thevehicle about said axis.
 9. Apparatus for stabilization about at leastone axis of an airborne vehicle operating in the electric field existingin the earth''s atmosphere, comprising: at least two static voltagesensing probes disposed on the airborne vehicle; means electricallyconnected to the probes for receiving the potentials sensed by saidprobes and for producing an output signal proportional to the differencein sensed potential between the probes; and, means for adjusting theposition of at least one of the probes to cause the output signal to bezero, the line joining the probes thereby being aligned along anequipotential line.
 10. The apparatus of claim 9 wherein the probes aredisposed along a line parallel to the roll axis of the airborne vehicle.11. The apparatus of claim 9 wherein the probes are located along a linedisposed at an angle to the roll axis of the airborne vehicle.
 12. Theapparatus of claim 11 wherein the angle is less than 45*.
 13. Theapparatus of claim 9 wherein the probes are disposed along a lineparallel to the pitch axis of the airborne vehicle.
 14. The apparatus ofclaim 9 wherein the probes are located along a line disposed at an angleto the pitch axis of the airborne vehicle.
 15. The apparatus of claim 14wherein the angle is less than 45*.
 16. The apparatus of claim 9 whereinsaid first-mentioned means comprises a differential amplifier. 17.Apparatus for stabilization relative to the surface of the earth aboutthe pitch and roll axes of an airborne vehicle operating in the staticelectric field existing in the earth''s atmosphere, comprising: twopairs of static voltage sensing probes disposed on the airborne vehicle,one said pair being disposed along a line substantially parallel to theroll axis of the vehicle and the other pair being disposed along a linesubstantially parallel to the pitch axis of the vehicle, the probes ineach pair of probes being disposed at spaced, co-planar points on thevehicle; two differential amplifiers, one each being electricallyconnected to one pair of probes for receiving the potentials sensed bythe probes of each pair of probes, each said amplifier producing anoutput signal which is proportional to the difference in sensedpotential between the sensing probes of each pair of probes; and, meansfor adjusting the position of at least one of the probes in each pair ofprobes to cause the output signal of the amplifiers respectivelyconnected thereto to be zero, the vehicle thus being stabilized relativeto the equipotential plane in which the probes lie when the outputsignals of both amplifiers are zero.
 18. The apparatus of claim 17 andfurther comprising: a pair of vertically disposed static voltage sensingprobes, one probe of the pair being disposed above the plane containingthe four aforementioned probes and the other probe being disposed belowthe aforesaid plane; means for receiving the potentials sensed by thevertically disposed probes and for producing an output signalproportional to the static voltage gradient between the probes; and,gain control adjustment means responsive to the last-mentioned outputsignal to provide automatic control of the gain required to render thefirst-mentioned output signals of sufficient magnitude to actuate themeans for adjusting the position of the probes in each pair of probes.19. Apparatus for detecting the attitude relative to the surface of theearth of an airborne vehicle operating in the electrostatic fieldexisting in the earth''s atmosphere, comprising: probe means mounted onsaid vehicle and responsive to the magnitude of said electrostatic fieldat a first preselected point relative to a second preselected point,said first and second preselected points being spaced apart and defininga reference line on said vehicle; and, means connected to said probemeans for producing an output signal proportional to the difference inrelative magnitude of said electrostatic field at said first and secondpreselected points, said output signal indicating misalignment betweensaid reference line and an equipotential line existing in said earth''selectrostatic field.
 20. The apparatus of claim 19 further includingstabilizing means on said vehicle responsive to said output signal foradjusting the attitude of said vehicle to cause said output signal to bezero and thereby align the reference line on said vehicle with saidequipotential line.
 21. The apparatus specified in claim 20, whereinsaid probe means comprises two pairs of static voltage sensing probesdisposed on the airborne vehicle, one said pair being disposed along aline substantially parallel to the roll axis of the vehicle and theother pair being disposed along a line substantially parallel to thepitch axis of the vehicle, the probes in each pair of probes beingdisposed at spaced, substantially horizontal co-planar points on thevehicle; wherein said output signal producing means comprises twodifferential amplifiers, one each being electrically connected to onepair of probes for receiving the potentials sensed by the probes of eachpair of probes, each said amplifier producing an output signal which isproportional to the difference in sensed potential between the sensingproBes of each pair of probes; and, wherein said stabilizing meanscomprises means for adjusting the position of at least one of the probesin each pair of probes to cause the output signal of the amplifiersrespectively connected thereto to be zero, the vehicle thus beingstabilized relative to the equipotential plane in which the probes liewhen the output signals of both amplifiers are zero.
 22. The apparatusof claim 21 and further comprising: a pair of vertically disposedvoltage sensing probes, one probe of the pair being disposed above thesubstantially horizontal plane containing the four aforementioned probesand the other probe being disposed below the aforesaid substantiallyhorizontal plane; means for receiving the potentials sensed by thevertically disposed probes and for producing an output signalproportional to the static voltage gradient between the probes; and,gain control adjustment means responsive to the output signalproportional to said static voltage gradient to provide automaticcontrol of the gain required to render the output signals produced bysaid differential amplifiers of sufficient magnitude to actuate themeans for adjusting the position of the probes in each pair of probes.23. The apparatus specified in claim 19 wherein said probe meanscomprises at least first and second electrostatic voltage sensor probesmounted on said vehicle at said first and second preselected pointsrespectively.
 24. The apparatus of claim 23 wherein the probes aredisposed along a line parallel to the roll axis of the airborne vehicle.25. The apparatus of claim 23 wherein the probes are located along aline disposed at an angle to the roll axis of the airborne vehicle. 26.The apparatus of claim 25 wherein the angle is less than 45*.
 27. Theapparatus of claim 23 wherein the probes are disposed along a lineparallel to the pitch axis of the airborne vehicle.
 28. The apparatus ofclaim 23 wherein the probes are located along a line disposed at anangle to the pitch axis of the airborne vehicle.
 29. The apparatus ofclaim 28 wherein the angle is less than 45*.
 30. The apparatus specifiedin claim 19 wherein said output signal producing means comprises adifferential amplifier.
 31. A method for detecting the attitude relativeto the surface of the earth of at least one axis of an airborne vehiclein the static electric field existing in the earth''s atmosphere,comprising the steps of: defining an equipotential line in theelectrostatic field existing in the earth''s atmosphere, which line issubstantially parallel to the surface of the earth; and, generating asignal indicative of misalignment between the axis of said vehicle andsaid equipotential line, and thus the misalignment between the axis ofthe vehicle and the surface of the earth.
 32. The method specified inclaim 31 and further including the step of aligning said axis of saidairborne vehicle with said equipotential line.
 33. The method specifiedin claim 31 and further including the steps of: defining a secondequipotential line existing in the electrostatic field to define anequipotential plane existing in said electrostatic field; and, aligningthe pitch and roll axes of the vehicle with said equipotential plane.34. The method specified in claim 31 wherein the step of defining saidequipotential line includes the step of: sensing said electrostaticfield with probe means responsive to the relative magnitude of theelectro-static field at two spaced points defining said vehicle axis.35. The method specified in claim 34, wherein the step of sensing therelative magnitude of said electrostatic field comprises the step ofsensing the electrostatic potential at each of two spaced pointsdisposed along a substantially horizontal line extending in the samedirection as the axis about which the vehicle is to be stabilized; and,wherein the step of generating said misalignment indicatiNg signalcomprising the step of generating a potential difference signalproportional to the magnitude of the difference in the two sensedpotentials, the magnitude of the signal being proportional to theangular misalignment of the aforesaid axis and an equipotential lineintersecting said axis.
 36. The method as specified in claim 35 andfurther comprising the steps of: measuring the magnitude of the verticalpotential gradient existing in the static electric field; and, adjustingthe gain of the potential difference signal in proportion to themagnitude of the measured vertical potential gradient.
 37. The method asspecified in claim 34, and further comprising the steps of: disposing atleast two electrically conductive members on the vehicle about the axisthereof, the members being electrically connected; and, generating asignal proportional to the current flowing between the conductivemembers on displacement of the vehicle about the axis, the magnitude ofthe current flow being proportional to the rate of angular displacementof the vehicle about the axis.
 38. A method for detecting the attituderelative to the surface of the earth of at least one axis of an airbornevehicle operating in the static electric field existing in the earth''satmosphere, comprising the steps of: sensing the electrostatic potentialat a first point in the atmosphere; locating a second point in theatmosphere having the same electrostatic potential as the first point todefine an equipotential line in the static electric field, theequipotential line being substantially parallel to the surface of theearth; and, generating a signal indicative of misalignment between theaxis of the vehicle and the equipotential line, and thus themisalignment between the axis of the vehicle and the surface of theearth.
 39. The method of claim 38 wherein the two points are disposedsubstantially along the line extending in the same direction as the axisabout which the vehicle is to be stabilized.
 40. The method of claim 38and further comprising the step of: aligning the axis of the airbornevehicle in response to the generated signal with the equipotential linethus defined.